cryptoexp

Crypto toolkit for CTF, security assessment and research — a pwntools-style toolbox of dependency-free primitives, plus an analysis pipeline on top.

Authors: coolmoon & guaidao2 — MIT. Import name and PyPI distribution name are both cryptoexp (the plain cryptokit name was already taken by an unrelated 2022 project, so this one is cryptoexp).

中文说明 · repository

• Library: import cryptoexp as cx — number theory, modular arithmetic, polynomial and GF(2) algebra, encodings, classic ciphers, block/stream ciphers, hash length extension, PRNG recovery, RSA/signature attacks, lattice tools, oracle attacks, and a forensics/assessment layer (batch GCD key audits, DER signature and JWT parsing, weak-PRNG fingerprinting).
• CLI: python3 cryptoexp_cli.py analyze|hypotheses|lab|list (analysis pipeline, JSON contract, solve-script generation).
• Zero dependencies: the core uses only the standard library (AES, LLL, Coppersmith, SHA-1/2 length extension, ChaCha20, MT19937 … are all implemented here). Optional accelerators (gmpy2, sympy, pycryptodome, z3) are auto-detected and never required.
• English-first source and tool output (reports, generated scripts, docstrings); this document is English, with a Chinese README alongside for readers who prefer it.
• from cryptoexp import * exists but is meant for throwaway solve scripts: it only brings names that are unmistakably ours. Generic primitives (gcd, sha256, AES, …) stay reachable as cx.gcd so a star import cannot shadow your own names.
• CTF solving is the core target, not the only one — see Beyond CTF.
• No web UI: this is a library plus a CLI.

Custom flag formats (library-first)

"What a flag looks like" belongs to the engagement, not to the library, so it is configurable in three layers. All of them work straight from pip install cryptoexp:

from cryptoexp import flag_candidates, find_flags, set_flag_prefixes

# 1) per call (no hidden state - the right way inside a library or a service)
flag_candidates(payload, prefixes=["DH", "corp_"])     # DH{...}, corp_...{...}
flag_candidates(payload, pattern=r"ACME-\d{4}-[a-z0-9]{8}")   # arbitrary format

# 2) process default (scripts, notebooks)
set_flag_prefixes(["DH"], merge=True)
flag_candidates(payload)                              # now uses DH, plus the built-ins

# 3) environment (CLI, CI)
#   CRYPTOEXP_FLAG_PREFIXES=DH,corp_ cryptoexp analyze challenge.txt

The CLI mirrors it: --flag-prefix DH (repeatable) and --flag-regex '<regex>'.

find_flags(data, prefixes=[...]) is the general entry point: it returns every marker-shaped string with {"match", "kind": "strict"|"loose", "offset"}, which is what you want on a dump, a config file or a log rather than on a challenge statement. Prefixes are matched literally (a prefix containing . cannot match abc{...}), matching is case-insensitive, and placeholder shapes such as DH{...} inside a statement are deliberately not treated as flags — otherwise a challenge would confirm itself.

analyze_all(path, flag_prefixes=["DH"]) scopes the format to that one run (a ContextVar, so parallel calls cannot leak into each other) and records it in the result, so verification and the JSON report agree with the caller.

Beyond CTF: security assessment and research

The same primitives answer real engagement questions. Library first, everything importable from the package root:

Honest scope, because overclaiming here is worse than a missing feature: cryptoexp does no network I/O, no scanning and no HTTP/TLS session parsing. Oracle attacks take a callable you supply (write a ten-line adapter for the target), and the analysis pipeline expects "a statement plus ciphertext blobs" as input. For research it gives you the primitives (LLL, Coppersmith, GF(2)/GF(p) algebra, DLP, PRNG state recovery, CRC/JWT/DER handling) as composable functions with published-vector tests.

Install

pip install cryptoexp          # once a stable release exists (see the alpha note)
pipx install cryptoexp         # if you only want the CLI

Every release so far is a pre-release (0.1.0aN), so pip needs --pre:

pip install --pre cryptoexp

From a clone, nothing needs installing — the package is importable through the repository entry point and the source tree:

git clone https://github.com/guaidao2/cryptoexp.git
cd cryptoexp
python cryptoexp_cli.py analyze challenges/rsa_wiener.txt     # CLI, no install
python -c "import sys; sys.path.insert(0, 'src'); import cryptoexp"   # or add src/ to PYTHONPATH

Zero runtime dependencies: the core is standard-library only, so it also works on locked-down machines without a package index.

Quick start

# nothing to install
python3 cryptoexp_cli.py analyze challenges/rsa_wiener.txt
python3 cryptoexp_cli.py analyze challenges/xor_repeating.txt --solve --run
python3 cryptoexp_cli.py hypotheses challenges/rsa_high_bits.txt   # attack surface + gaps
python3 cryptoexp_cli.py lab "paste the whole statement here"      # workbench scaffold
python3 cryptoexp_cli.py list                                      # analyzers + solvers + API map

import cryptoexp as cx

cx.long_to_bytes(0x4142)              # b'AB'
cx.b64d('ZmxhZ3thfQ==')               # b'flag{a}'
cx.algebra.gcd(24, 36)                # 12
cx.lll([[1, 1, 1], [1, 0, 1], [0, 1, 1]])         # pure-Python lattice reduction
cx.known_high_bits_factor(n, p_high, 160)         # Coppersmith
cx.wiener_attack(e, n, c)                         # {'ok':..., 'plaintext':..., 'factors':...}
cx.ecb_byte_at_a_time(cx.make_ecb_oracle(key, secret))   # oracle attacks
cx.padding_oracle_attack(valid, iv + ct)
cx.clone_from_outputs(six_hundred_and_twenty_four_words) # MT19937 state clone
cx.discrete_log(g, h, p)['x']                     # BSGS -> Pohlig-Hellman
cx.dsa_nonce_reuse(p, q, g, y, r, s1, s2, h1, h2) # nonce-reuse key recovery
cx.crc_compute(b"123456789", 32, 0x04C11DB7, 0xFFFFFFFF, True, True, 0xFFFFFFFF)  # 0xCBF43926
cx.length_extension(h1, len(data), b"&admin=1", secret_len=len(secret))
cx.audit_rsa_key(n, e, extra_moduli=[other_n])    # weak-key findings with severity
cx.berlekamp_massey(bits)                          # recover LFSR taps from output bits
cx.java_random_predict(outputs, 4)                 # java.util.Random state recovery

Coverage (the mechanical operations)

Why it is not a template bank

intelpwn's "fixed vulnerability class → fixed exploit chain" works for pwn because the classes are finite. Crypto is the opposite: you read the construction, find the structural weakness, and often write the last step. So cryptoexp is layered:

Templates are a convenience, not the answer. Mechanical operations live in the library so you can compose them; for the long tail the CLI hands you the ranked hypotheses, the missing-information list and a workbench.

Design notes (inherited discipline)

• Blackboard: analyze_all materialises the context once (text, files, named parameters, ints, blobs, decoded bytes); analyzers consume it instead of re-parsing. Failures degrade to warnings and never abort the run.
• Two registries: register_analyzer(name) and register_solver(name, predicate, gen, priority); unknown keys render automatically, so a new analyzer needs no changes in the presentation layer.
• Evidence grading: every conclusion carries severity + confidence; "rule guess" is labelled as such.
• Unknown is a first-class state: hypotheses declare what they need and the engine reports the gap instead of staying silent.
• Bounded work: expensive steps take explicit budgets; a budget hit is reported.
• Verification beats heuristics: every bug found while building this is now a regression test. Examples: search objectives must not contain the flag bonus (hill climbing otherwise fabricates a flag), Coppersmith must reject the trivial p = n "factor", LLL parameters m=t=3 are required for 160-of-256 known bits, generated scripts must actually run and print the flag.

Tests

python -m unittest discover -s tests -v
python challenges/make_challenges.py     # regenerate the sample corpus (optional)

tests/test_library.py checks official vectors (FIPS-197 AES, RFC 8439 ChaCha20, CRC-32/CRC-16 check values, SHA-1/SHA-256 test vectors), algebra and lattice identities, oracle attacks against locally built oracles, signature nonce reuse and the MT19937 clone. tests/test_toolkit.py covers the mechanical layer added in v0.3: modular symbols and non-coprime CRT, polynomial/GF(2) arithmetic, CRC forgery, LFSR tap recovery, length extension, RC4/ChaCha20/AES-CTR vectors, Java/glibc PRNG recovery, RSA attack wrappers, key parsing and the signature primitives. tests/test_challenges.py is end-to-end: 19 samples must yield their flags, generated solve scripts must compile and run to the flag, and the JSON contract must hold. tests/test_forensics.py covers the assessment layer (batch GCD, key audit, JWT/DER parsing, weak-PRNG identification) on constructed real-world material.

Current total: 153 tests, all green (python -m unittest discover -s tests).

Honest limitations

• glibc_rand_recover needs roughly 96+ consecutive outputs. Below that the hidden low bits are genuinely underdetermined: the function enumerates all states consistent with the observations and returns a state only when they agree on the next outputs, otherwise None. It never guesses.
• lcg_recover_truncated is not implemented as a lattice. When the visible high bits do not uniquely determine the parameters it returns None (documented in its docstring) instead of inventing a sequence. The lattice formulation was attempted and did not reach a reliable state.
• Coppersmith with degree > 1 only works at small lattice sizes in pure Python: stereotyped_message and hastad_padded therefore try the exact integer-root path first (when no reduction modulo n happens, which is the common CTF shape) and fall back to the lattice, which is where the time budget can run out on large moduli.
• ECC beyond toy curves, multivariate Coppersmith, Boneh-Durfee, Bleichenbacher's full attack: documented skeletons or absent — the tool reports the hypothesis and the gap rather than pretending.
• Discrete log only via BSGS / Pohlig-Hellman; group orders with large prime factors are reported as infeasible.
• Pure-Python Fraction LLL is comfortable up to dimension ~8; Coppersmith defaults for degree 1 are tuned to m=t=3 (~6 s on a 512-bit N with 160 known bits).
• Oracle attacks need a callable; the workbench ships a remote-oracle template.
• The analyzer routes CRC and LFSR challenges end to end (catalog lookup, preimage, forgery; tap recovery + keystream) with generated solve scripts. The remaining new families (ChaCha20/RC4, hash length extension, Java/glibc PRNG, keys, signatures) are library + unit-test coverage with published vectors, not yet routed by the CLI: compose the library calls (or use the hypothesis/workbench layers) for those.
• CRC preimage solving pins the unknown field only when 8 * unknown_len <= width (4 bytes under CRC-32, 2 under CRC-16). With more unknowns it returns one solution with the CRC matched and says unique: False — several byte strings fit, so it cannot claim to have found the original one.
• Playfair / Hill / columnar / bacon are lossy by design (X padding, I/J and U/V folding), so "round-trip" there means decrypt(encrypt(x)) == prepared(x).
• No web interface and no CI yet.

License: MIT — coolmoon & guaidao2.